1. Field of the Invention
This invention relates to a method and an apparatus for producing a picture by dots and, more particularly, to a method and an apparatus for producing a picture in which the reproduction of a density gradient is possible.
2. Description of the Prior Art
Although various kinds of apparatus which can reproduce density gradients have been conventionally proposed, the present invention will be described in detail hereinbelow with respect to an example of an ink jet printer.
The following methods of reproducing the density gradient have been proposed conventionally.
A first method is to reproduce the density gradient by varying the dot diameter size to be printed by controlling the quantity of liquid to be discharged from the ink jet head.
As a second method, the dot diameter is not changed but one picture element (i.e. pixel) is constituted by a 4.times.4 matrix consisting of, for example, micro-elements of and the density gradient is reproduced using a dither process for this matrix.
However, according to the first method, it is difficult to take a wide range from the minimum dot diameter to the maximum dot diameter and thus only the reproduction of the order of several density gradients can be done. Thus, this method is unsatisfactory for the printout of television pictures or photographs, or the like.
The second method improves over in the first method, and if one pixel is represented by the matrix of 4.times.4, seventeen density gradients can be reproduced. However, an increase of one pixel by 16 times (4.times.4) than that in the first method causes the print speed to be reduced by 1/16, or the print speed must be made fast by increasing the number of print heads by 16 times. However, this countermeasure not only makes the construction of the print head complicated but also greatly increases the complexity of the electric circuit for processing pictures by means of the dither process, resulting in an extreme increase in overall cost.